Hierarchical ZSM-5 beads composed of zeolite nanosheets obtained by pseudomorphic transformation

Abstract

The concept of pseudomorphic transformation was used to transform amorphous mesoporous silica beads with different size (20, 50 and 75 μm) into hierarchical MFI-type zeolite beads composed of ZSM-5 nanosheets. The beads were synthesized under hydrothermal conditions with and without mechanical stirring at different temperatures and treatment times. The influence of the different synthesis parameters was investigated by X-Ray Diffraction, Scanning Electron Microscopy, Transmission Electron Microscopy, X-Ray Fluorescence, Nitrogen adsorption-desorption measurements, 27Al solid-state Nuclear Magnetic Resonance, Energy-Dispersive X-Ray analysis and Thermogravimetric analysis. Well-crystallized 20 μm ZSM-5 nanosheets beads similar in size and shape to the original mesoporous amorphous silica beads were obtained after a hydrothermal treatment at 150 °C for 5 days in a tumbling oven and 2 days in static conditions at 120 °C. The influence of the bead size on the pseudomorphic transformation was studied in order to prepare well-crystallized ZSM-5 nanosheets beads of 50 and 75 μm. Results showed that the required time of static treatment at 120 °C increases when the size of the parent silica spheres increases. This is explained by the fact that crystallization starts from the outer bead surface toward the center.